Heretofore, there has been proposed a technology that determines whether or not the combustion apparatus is in a normal combustion mode according to whether or not the detected combustion exhaust temperature falls within the range between the upper and the lower limits found by calculation (see, for example, Patent Literature 1). In addition, there has been proposed another technology that determines whether or not to perform a safety operation for preventing the exhaust duct from firing according to whether or not the combustion exhaust temperature, detected midway during directing of the flow of combustion exhaust from the exhaust hood of the combustion appliance to the main duct channel through which combustion exhaust is discharged to the outside, falls within the predefined allowable combustion exhaust temperature range (see, for example, Patent Literature 2).